Chlorine dioxide is used as a bleach in a variety of environments, notably in the bleaching of wood pulp. Various chemical processes for the generation of chlorine dioxide by reduction of sodium chlorate in aqueous acid media have been described in the past and are in commercial operation. The chemical process may be depicted by the equation (I): EQU 2ClO.sub.3.sup.- +2Cl.sup.- +4H.sup.+ .fwdarw.2ClO.sub.2 +Cl.sub.2 +2H.sub.2 O (I)
U.S. Pat. No. 4,426,263 (Hardee et al) describes an electrolytic process for producing chlorine dioxide using an electrocatalyst comprising a platinum group metal oxide as a cathode coating in an electrolytic cell containing sodium chlorate and sulphuric acid. This patent also describes the use of platinum group metal oxides as a catalyst in the absence of applied electrical current and indicates that this is the preferred embodiment.
Since the material used as the cathode coating is itself a catalyst for the production of chlorine dioxide, the effect of an applied current cannot readily be determined but the data presented in the patent and also in an article by Hardee describing the electrochemical process (see "The Electrochemical Generation of Chlorine Dioxide Utilizing Electrolytic Oxide Coatings", Extended Abstracts, vol. 85-1, pp.617 to 618, The Electrochemical Society, 1985) suggest little beneficial effect of the applied current on the generation of chlorine dioxide.
According to the article, the better efficiencies are observed at lower current values and hence at lower contributions of electrolysis to the overall process of generation of chlorine dioxide. In particular, current efficiencies as low as 20% were observed at higher current densities. The observed loss in efficiency was believed to arise from further reduction of chlorine dioxide.
Poor results obtained in electrolytic experiments carried out at higher current densities are in a good correspondence with the cyclic voltammograms reported in the above-noted Hardee article, where the maximum current density observed for the electroreduction of 0.5M NaClO.sub.3 is less than 10 mA/cm.sup.2, which is, by an order of magnitude, lower than expected for such a high concentration of reducible species.
The experimental data in the Hardee article indicates to one skilled in the art that the process is not limited by the electrochemical step involving chlorate but rather by a chemical step in which an electroactive species, different from chlorate, is formed, which undergoes subsequently an electroreduction. Accordingly, the rate of reduction of chlorate ion to chlorine dioxide is limited by a chemical reaction rather than an electrochemical one and this chemical reaction can be accelerated by the presence of a catalyst, as described in the Hardee patent and article. Although the platinum metal oxide catalyst appears to enhance the rate of chemical conversion of chlorate ion to chlorine dioxide, its electrocatalytic properties have a detrimental effect on the electrochemical stability of the desired product, namely chlorine dioxide, when practical current densities are applied to electrodes having a surface of such platinum metal oxides.
The Hardee article also claims that the platinum group metal oxides are the only materials which show activity for the reduction of chlorate and data is presented showing the alleged ineffectiveness of platinum.